GnuTLS is too strict on the SSL shutdown alert, and it's neither
mandatory in the spec or critical. As it's ignored in OpenSSL, we
should also suppress it in GnuTLS as well.

Ticket: #6667
Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>